1. Field of the Invention
The present invention relates to signal processing. More specifically, the present invention relates to a signal processing method and apparatus which uses time-domain digital signal processing for removal of clutter from a signal.
2. Background of Related Art
Pulse-echo ultrasonic imaging technology has become a vital tool for clinicians for examining the internal structure of living organisms. In the diagnosis of various medical conditions, it is often useful to examine soft tissues within the body to show structural details of organs and blood flow in these organs. Experienced clinicians can use this information in diagnosing various pathologies.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties of the echoes (e.g., amplitude and phase) from targets at varying depths within the body. Typically, the ultrasound beam is focused at various depths within the body in order to improve resolution or image quality. The echoes are received by a transducer, typically the same transducer used for transmission, and processed to generate an image of the object, usually referred to as a B-scan image.
Measuring and imaging blood flow (or other fluid flow) in the human body is typically done using the Doppler principle, wherein a transmitted burst of ultrasound at a specific frequency is reflected from moving blood cells, thereby changing the frequency of the reflected ultrasound in accordance with the velocity and direction of the flow.
The frequency shift of these reflected signals with respect to the transmitted signals may be detected and since the amount of the shift (or the Doppler shift) is proportional to the blood flow velocity, it may be used to display velocity information of blood flow on a video screen for imaging the living patient.
Undesirable signals, which are commonly called clutter (or sometimes flash artifacts) often arise from structures and targets in the body which may have Doppler shifts, or from movement of the transducer by the operator, but do not represent blood or fluid flow. Examples of these clutter signals include tissue motion due to the heart beat, arterial pulse or respiration. Such unwanted signals are typically filtered out or otherwise processed, so that the image produced only represents blood flow and clutter is suppressed.
In color Doppler imaging this processed frequency information is used to form a two-dimensional image or profile of the blood or fluid flow velocity.
A typical ultrasound system for color Doppler imaging is shown in FIG. 1 as imaging system 100. Imaging system 100 generally comprises a probe 101, which is typically a multi-element array of one hundred or more piezoelectric elements which both send and receive ultrasound signals when examining the human body. Probe 101 is coupled via signal path 110 to transmitter/receiver circuitry 102, which is designed according to principles well known in the ultrasound imaging art and for purposes of brevity will not be discussed in detail here.
Transmitter/receiver circuitry 102 is coupled to a control unit 109 via bus 120 and is controlled so that the elements in probe 101 are focusing at particular points in the body, both on transmit and receive. Transmitter/receiver circuitry 102 and control unit 109 also often provide a scanning function such that a two dimensional image may be generated without moving probe 101 with respect to the body.
Following transmission of ultrasound signals into the body, reflected signals are processed by the receiver function (which is typically known as a beamformer) in transmitter/receiver circuitry 102 and the multitude of signals from each individual element of probe 101 are converted into a single signal which is sent to RF (Radio Frequency) processor 103 via signal path 111.
RF processor 103, also under the control of control unit 109 via bus 120, processes the signal information to produce a detected and unipolar envelope signal and in-phase (I) and quadrature (Q) Doppler signals. The envelope signal represents the amplitude of echoes returning from the body and is further transmitted via signal path 114 to a scan converter 105 which is a typically a large electronic memory, also well known in the art.
Scan converter 105, also under the control of control unit 109 via bus 120, stores the envelope echo information on a line by line basis together with the geometrical position of such information in the body resulting from the scanning process, in such a manner that a two-dimensional video image may be constructed and transmitted to video processor 127 via signal path 116. Video processor 127 is also under the control of control unit 109 via bus 120.
In the absence of any color Doppler information, video processor simply sends a conventional video signal over signal path 119 to video display monitor 130. This two-dimensional image, usually black and white, represents the distribution of echo generating sites within the body. The so-called B-scan image is thus used by the operator to search the body for pathology or by the physician in developing a diagnosis.
I and Q signals for so-called single-gate Doppler are sent to Doppler processor 106 via signal path 113. Doppler processor 106, under the control of control unit 109 via bus 120, using signal processing methods well known in the art, compares signals from several successive echoes to determine the Doppler shift in a single region in the body which is commonly known as the sample volume. Doppler processor 106 also typically produces a continuous time series of spectral Doppler information in which blood flow velocities are displayed in black and white on video display 130 over one or more cardiac cycles (typically several seconds), having first been sent to scan converter 105 via signal path 115, to video processor 127 via signal path 116 and to video display 130 over signal path 119.
Finally, the third path to video display 130 is the color Doppler path in which the preferred embodiment may effect the signal, as discussed below.
RF processor 103 transmits I and Q signals via signal path 112 to color flow processor 104 which is also controlled by control unit 109 via bus 120. Color flow processor 104 typically processes several Doppler sample volumes along a given scanning direction in the body. Details of prior art color flow processing will be discussed below.
Color flow processor passes signals to color scan converter 108, also under the control of control unit 109 via bus 120, via signal path 117 where, in a manner similar to the black and white scan converter 105, color encoded signals are stored on a line by line basis, together with the geometrical position of such information in the body resulting from the scanning process, in such a manner that a two-dimensional color video image may be constructed and transmitted to video processor 127 via signal path 118.
Color scan converter 108, which may also be used to interpolate scan line information obtained from color flow processor 104, then transmits color Doppler information via signal path 118 to video processor 127 for display on video display 130. Video processor 127 typically includes so-called decision circuits to choose whether a given specific part of the two dimensional image has color information resulting from flow or whether it only has echo information from static tissue. If flow is present, the color information is displayed at the correct point in the image rather than the black and white image information.
This final composite two-dimensional color image showing blood flow in color overlaid on a black and white image represents the velocity of blood flow in vessels or organs and is used by the clinician to form a diagnosis of flow related pathology.
Control unit 109 is further coupled to a keyboard 125 for operator inputs and a mouse, trackball or other device 126 for movement and control of information shown on video display 130.